A sonar (or radar) system that is capable of continuously transmitting and receiving Continuous Wave (CW) signals can determine a vehicle's speed by measuring the Doppler shift but cannot measure range. If the continuous transmission is frequency modulated and the vehicle is stationary, the range to the vehicle may be determined by measuring the frequency difference between the transmitted signal and the received echo. This concept is used quite extensively in altimeters, police radars, Doppler navigators, and personnel motion detectors (Radar Handbook, McGraw-Hill, M. I. Skolnik, editor). Past techniques to obtain the difference frequency have used the transmitted signal waveform as the demodulating signal.
However, if the vehicle is moving, the instantaneous difference between the transmitted and received echo is dependent on range, range rate and, to a lesser degree, time. The interdependency results in range Doppler ambiguity found in conventional continuously transmitted frequency modulated (CTFM) sonar and radar systems. While Pulse-Echo (PE) sonar/radar systems have been developed for determining speed and range of target vehicles, such systems typically require significantly higher peak power than CTFM systems with equivalent detection and localization performance because the PE system typically operate at low duty cycle whereas as a CTFM system works at or near 100% duty cycle. Thus, CTFM can have the advantage of requiring substantially lower transmitted power and correspondingly smaller transmitters than equivalent performance PE systems. A CTFM system can also provide faster update rates. Thus, a CTFM system that can resolve the range Doppler ambiguity can have significant advantages over known PE systems.